Which of the three in the 2nd picture is the "low limit screw"? And how to be sure to set it correctly? (I'm bringing screw driver with me this afternoon just in case)
On Shimano deraillers it's typically the one that is physically lowest when you're viewing the bike upright. It will generally have a very small 'L' next to it.
For adjustments, follow the steps at
http://www.parktool.com/blog/repair-...nts-derailleur .
It shouldn't be a matter of "just in case" - you really need to get them right before you ride the bike. You may be lucky and shift successfully 20 times before you damage the wheel, or it may take just once. The high limit screw is important as well, but generally the worst that happens if that one is too free is that you jam the chain between the cogs and frame, leading to an abrupt and unexpected stop.